Some automobiles utilize an internal combustion engine to provide at least a portion of the wheel power requested by the driver. As one example, a hybrid electric vehicle (HEV) can control the engine to operate under select conditions, while an electric motor can separately provide the requested propulsive effort. While some efficiency gains may be achieved by supplementing the operation of the engine with that of a motor, substantial inefficiencies may nevertheless remain. As one prophetic example, up to 75% of the fuel energy consumed by an internal combustion engine may be lost to unexploited thermal energy, under some conditions. Of this, approximately 35% may be lost to the exhaust gases of the engine, 20% to the coolant or cooling process, and another 20% to thermal radiation.
Some vehicle propulsion systems have attempted to address the above issues by reducing the amount of time that the engine is operating or by downsizing the engine by increasing use of the motor. Other approaches have sought to confront these inefficiencies directly by attempting to recapture the rejected thermal energy through the use a thermoelectric conversion device. For example, a vehicle may include one or more of these devices for converting at least a portion of the thermal energy of the exhaust gases produced by the engine to electrical energy for use by other vehicle systems.
These thermoelectric devices rely on a temperature gradient to generate electrical energy. As such, efforts have been made to subject the thermoelectric device to as great of a temperature gradient as possible. However, one issue with the thermoelectric device includes the potential for thermal damage caused by excessive temperatures. Thus, while large temperature gradients may be useful to efficiently recapture rejected thermal energy, if temperatures are too high, protection of the device from excessive temperature conditions may occur.
Various approaches have attempted to provide an acceptable yet efficient operating environment for the thermoelectric device, including the use of variable heat exchangers arranged between the heat source and the device. However, the additional hardware can increase the cost of the vehicle, reduce efficiency via increased weight or pumping losses or create additional issues, under some conditions.
Additionally, the inventors of the present disclosure have also recognized that some portions of the propulsion system such as the engine and/or catalyst may experience reduced efficiencies during cooler operating conditions, such as before or during warm-up of the engine.
In one approach, the above issues may be addressed by a propulsion system for a vehicle, comprising an internal combustion engine; a thermoelectric device in thermal communication with the engine; and a control system for controlling the engine and the thermoelectric device to during a first mode, supply electrical energy to the thermoelectric device to cause the thermoelectric device to produce at least some heat; and during a second mode, operate the engine to produce at least some heat, where the thermoelectric device is operated to convert a temperature gradient at the thermoelectric device to electrical energy.
In this way, the temperature of the engine, exhaust passage, and/or catalyst may be controlled, for example, by providing heating during lower temperature conditions, while electrical energy generation may be achieved during higher temperature conditions.
Further, the above system may be used in combination with a free-piston engine to achieve even greater efficiency gains over fixed-piston type engines having pistons mechanically coupled to a crankshaft, while an induction device can be used to convert kinetic energy of the free-piston to electrical energy that may be combined with or used separately from the energy generated by the thermoelectric device to power a drive motor.
In this way, synergy between the free-piston engine in a full series or other HEV configuration and the thermoelectric conversion system may be achieved by varying operation of the free-piston engine in response to the temperature at the thermoelectric devices. The thermoelectric devices may then be placed closer to the heat source, such as directly at or surrounding the engine where the temperature may be controlled within a more efficient temperature range while reducing high temperature conditions that may otherwise occur. Note that variable heat exchangers arranged between the heat source and the device may additionally be used, along with fixed piston engines, if desired.